[otci] address small reliability issues (#7372)

This commit addresses several small reliability issues revealed during
testing.

tools/otci/otci/connectors.py

@@ -159,7 +159,7 @@ class OtCliSerial(OtCliHandler):
                 line = self.__linebuffer + line
                 self.__linebuffer = b''
 
-                return line.decode('utf-8').rstrip('\r\n')
+                return line.decode('utf-8', errors='ignore').rstrip('\r\n')
 
         return None
 

tools/otci/otci/otci.py

@@ -47,7 +47,9 @@ class OTCI(object):
     manipulate an OpenThread device.
     """
 
-    __exec_command_retry = 0
+    DEFAULT_EXEC_COMMAND_RETRY = 4  # A command is retried 4 times if failed.
+
+    __exec_command_retry = DEFAULT_EXEC_COMMAND_RETRY
 
     def __init__(self, otcmd: OTCommandHandler):
         """
@@ -278,7 +280,12 @@ class OTCI(object):
         for line in output[2:]:
             fields = line.strip().split('|')
 
-            _, J, netname, extpanid, panid, extaddr, ch, dbm, lqi, _ = fields
+            try:
+                _, J, netname, extpanid, panid, extaddr, ch, dbm, lqi, _ = fields
+            except Exception:
+                logging.warning('ignored output: %r', line)
+                continue
+
             networks.append({
                 'joinable': bool(int(J)),
                 'network_name': netname.strip(),